Method for operating an internal combustion engine, and corresponding device

ABSTRACT

A method and a corresponding apparatus for operating an internal combustion engine are proposed, in which in at least one cylinder, combustion of a fuel in a fuel/air mixture introduced into the cylinder takes place. Upon the occurrence of knocking, a baseline ignition angle in the at least one cylinder is retarded in steps, using a knock control system, to an adjustment angle from the basic ignition angle. When the adjustment angle exceeds a definable threshold value, activation of the knock prevention operation mode occurs, in which context an additional action reducing the knock susceptibility of the fuel/air mixture is taken and the adjustment angle is decreased so that the baseline ignition angle is advanced.

BACKGROUND INFORMATION

[0001] The invention is based on a method for operating an internal combustion and a corresponding apparatus, according to the species defined in the independent claims. A method for operating an internal combustion engine and a corresponding apparatus having at least one cylinder are already known; in that context the ignition angle in the at least one cylinder is displaced from the engine-speed-dependent or load-dependent basic ignition angle on the basis of utilization of a knock control system. The adjustment angle depends on the frequency of knocking in the at least one cylinder, and is greater, the more frequently the knock signal has occurred in the previous working cycles.

ADVANTAGES OF THE INVENTION

[0002] The method according to the present invention and the apparatus according to the present invention having the features of the independent claims have, in contrast, the advantage that the reduction in torque caused by the ignition retard resulting from the knocking can be decreased. It is thus possible to advance the ignition angle once again and thus achieve greater torque, and thereby increase the output of the internal combustion engine. Better efficiency can thus be achieved.

[0003] The features set forth in the dependent claims make possible advantageous developments of and improvements to the method and the apparatus recited in the independent claims. One particularly simple action in the case of a direct gasoline injection internal combustion engine, in order to advance the ignition angle, is to perform a double injection operation in one working cycle of a cylinder. By way of a first injection, a lean basic mixture is first formed in the respective cylinder, into which injection then occurs a second time. The lean basic mixture that is still present in peripheral zones of the mixture even after the second injection reduces the knock susceptibility. In addition, the fuel delivered to the combustion chamber by the second injection cools the fuel/air mixture, which results in a further reduction in knock susceptibility. It is moreover particularly advantageous, when a double injection is used, not to modify the volume of fuel to be injected as compared to a single injection, so that less fuel is consumed in terms of the achievable output. Equally easy to achieve as an additional feature is a change in the exhaust gas recirculation rate, if an exhaust gas recirculation system is used in the internal combustion engine. At a higher exhaust gas recirculation rate, the knock susceptibility is also reduced because of a decrease in the oxygen content of the fuel/air mixture.

[0004] It is furthermore advantageous to employ as the representative adjustment angle the mean of all individual-cylinder retard angles of the cylinders, since an error-prone switchover is thereby prevented. On the other hand, however, it is also advantageous to employ the individual-cylinder retard angles as adjustment angles, and to carry out activation of the knock prevention operating mode in individual-cylinder fashion, so that a very accurate reaction to the individual cylinders' properties can take place.

[0005] It is furthermore advantageous to carry out the knock prevention with double injection operating mode only above a definable first load threshold, since application of the knock prevention with double injection operating mode is desirable only in a specific load range. Upon switching back into another operating mode when the load falls below a specific threshold value, it is desirable to select that second load threshold value in such a way that it lies below the first load threshold value, in order to prevent an unstable state and continual switching back and forth between two operating modes. Since both the first load threshold value and the second load threshold value can be different depending on the engine speed, it is desirable to take this engine speed dependence into account.

[0006] For the knock prevention with exhaust gas recirculation operating mode as an additional action reducing the knock susceptibility, it is desirable to access this only below a definable third load threshold value, since the exhaust gas recirculation feature yields no torque gain at higher loads. By analogy with the knock prevention with double injection operating mode, the intention is to eliminate unstable states between two operating modes, so that advantageously the fourth load threshold value, at which operation is switched back from the knock prevention with exhaust gas recirculation operating mode into another operating mode, should lie above the second load threshold value. Since both the third load threshold value and the fourth load threshold value can be different depending on the engine speed, it is desirable to take this engine speed dependency into account, by analogy with the knock prevention with double injection mode.

[0007] It is furthermore advantageous, upon activation of the knock prevention operating mode, also to take as the basis for the basic ignition angle another load-dependent and/or engine-speed-dependent characteristics diagram specific to that operating mode, since the conditions in the combustion chamber and thus the ignitability of the fuel/air mixture have been greatly changed by the additional action.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0008] Knocking, i.e. an abnormal combustion process in the combustion chamber of an internal combustion engine, limits output and efficiency and results in damage to the combustion chamber because of the high flame velocities. To prevent this, systems for knock detection and knock control are integrated into modern internal combustion engines. Systems of this kind are sufficiently known from the literature, and their manner of operation will therefore merely be summarized briefly below.

[0009] By the knock sensors belonging to the knock detection system, signals are acquired from the combustion chamber and are forwarded to an analysis unit also belonging to the knock detection system. Signals of this kind can be, for example, acoustic signals, electromagnetic signals, electrical signals, pressure signals, or temperature signals, which are received inside or outside the combustion chamber by the knock sensors. The analysis unit performs an amplification of the signals and integrates the signals in suitable fashion, so that after an analog/digital conversion, the signals of the integrated values can be compared, in a microprocessor contained in the internal combustion engine, to a threshold value, in which context knocking can be detected if the threshold is exceeded. In addition, for example, a selection of a specific frequency band or a selection in terms of a specific time window can be performed in the analysis unit in order to suppress interference signals, the frequency band or time window being characteristic for the occurrence of knock signals.

[0010] If knocking has been detected, the knock control function is then performed by the microprocessor integrated into the internal combustion engine, in that an adjustment angle is added to a basic ignition angle, the sum constituting the baseline ignition angle. From the torque structure, a torque-based ignition angle is calculated on the basis of the torque requests; this represents the earliest possible ignition angle. In the method for calculating the ignition angle, the baseline ignition angle and the torque-based ignition angle are then compared to one another, in which context the later of the two ignition angles, i.e. the ignition angle closer to top dead center, serves as the ignition angle to be implemented. If the fuel/air mixture in the combustion chamber has a strong tendency to knock, the adjustment angle will assume large values; in the context of this Application, the adjustment angle is defined as a positive value, so that the ignition angle is greatly retarded, i.e. ignition occurs at a time closer to top dead center. The basic ignition angle is read out from a load-dependent and/or engine-speed-dependent characteristics diagram that contains ignition angles under standard conditions and is stored in the microprocessor of the internal combustion engine. It contains values for the basic ignition angle that are associated with specific engine speed and/or load ranges. The engine speed, in this context, is ascertained by sensors that preferably are mounted on the crankshaft. The load, i.e. the relative air charge of the particular cylinder, is ascertained by the microprocessor using models, on the basis of a variety of operating parameters such as engine speed and torque request, and on the basis of readings of the hot film air mass sensor (HFM) and the intake manifold pressure sensor, the exhaust gas recirculation rate, or the position of the throttle valve, and is made available for the determination of the basic ignition angle. The relative air charge is defined as the ratio between the instantaneous air charge and the air charge under standard conditions for the particular cylinder. Adjustment of the ignition angle by the knock control system is performed until no further knocking occurs. The adjustment of the ignition angle in the individual steps adds up to an adjustment angle that indicates the total adjustment of the ignition angle with reference to the basic ignition angle. If no further knocking occurs for a specific period of time, the ignition angle is then once again advanced by the knock control system, i.e. the ignition angle is located at crankshaft angles farther away from top dead center. The adjustment angle becomes correspondingly smaller as a result of the advancing of the ignition angle. A disadvantage of the conventional knock control method is that when the ignition angle is greatly retarded, the achievable torque and therefore efficiency are greatly decreased.

[0011] The method according to the present invention for operating an internal combustion engine is characterized, in contrast, by a new operating mode that can be carried out both in a direct gasoline-injection internal combustion engine and in an internal combustion engine with manifold injection. The method is performed by a microprocessor located in the internal combustion engine. If the adjustment angle of the knock control system is greater than a specific definable threshold value, the microprocessor then activates a knock prevention operating mode which encompasses the fact that an action reducing the knock susceptibility is performed, and the adjustment angle is correspondingly modified in such a way that the baseline ignition angle is advanced. A torque gain, and thus an increase in efficiency, can thereby be achieved.

[0012] In a first exemplary embodiment of the invention, two injection operations are performed in one working cycle of a cylinder as the action reducing the knock susceptibility in a direct gasoline-injection internal combustion engine. The performance of two injection operations in one working cycle is also referred to as “double injection.” One working cycle encompasses the cycle of the four strokes of an engine using the four-stroke process—intake stroke, compression stroke, power stroke, and exhaust stroke—or the two strokes of an engine using the two-stroke process—scavenging and compression stroke, and power stroke. In this exemplary embodiment, the modification of the adjustment angle is performed in such a way that the adjustment angle is decreased, which means that the baseline ignition angle is advanced. Injection of fuel into the combustion chamber twice in one working cycle is carried out in such a way that during the intake stroke or the scavenging and compression stroke, a homogeneous lean basic mixture is created by way of a first injection. Into this homogeneous basic mixture, shortly before ignition (i.e. in the compression stroke or also in the scavenging and compression stroke), a rich cloud is placed around the spark plug by a second injection. This stratification guarantees reliable ignition of the fuel/air mixture in the combustion chamber, while at the same time knock susceptibility is reduced because of the leaner edge zones. A reduction in knock susceptibility is also achieved by the fact that the combustion chamber is cooled by the second injection of fuel. In this context, the total fuel quantity is not increased as compared to a single injection, but rather the fuel quantity is distributed over the two injection operations. Because of the greater torque achievable as a result of the earlier ignition angle, it is thus possible to attain greater efficiency with the same fuel consumption, thereby reducing the consumption of the internal combustion engine as a function of output.

[0013] In a further preferred exemplary embodiment, the action reducing the knock susceptibility encompasses an increase in the exhaust gas recirculation rate, if an exhaust gas recirculation system is integrated into the system of the internal combustion engine. This action can be performed both in a direct gasoline-injection internal combustion engine and in an internal combustion engine with manifold injection. In this context, exhaust gas recirculation includes both internal and external exhaust gas recirculation, the term “internal exhaust gas recirculation” being applied to the operation in which the exhaust gas in the cylinder is not completely expelled, but instead exhaust gas remains in the cylinder and is once again a constituent of the fuel/air mixture in the cylinder in the next working cycle. “External exhaust gas recirculation” encompasses the recirculation of exhaust gas from the exhaust system into the intake manifold of the internal combustion engine. The external exhaust gas recirculation rate is influenced by way of the exhaust gas recirculation valve, which in turn is controlled by the microprocessor. The internal exhaust gas recirculation rate can be influenced by the camshaft setting. An increase in the exhaust gas recirculation rate decreases the oxygen content of the fuel/air mixture present in the cylinder, making it difficult for abnormal combustion processes to occur in the combustion chamber.

[0014] In a further preferred exemplary embodiment, the basic ignition angle is taken from another characteristics diagram, specific to the knock prevention operating mode, that is also stored in the microprocessor. The specific characteristics diagram is different depending on the knock susceptibility-reducing action that is taken. The modified characteristics diagram for the knock prevention operating mode takes into account the modified conditions for ignition of the fuel/air mixture in the combustion chamber as a result of the double injection or the modification of the exhaust gas recirculation rate. In this preferred exemplary embodiment, during the knock prevention operating mode the knock control and knock detection functions continue to be performed. This means that the adjustment angle, which has been modified by the transition into the knock prevention operating mode, continues to be adapted in accordance with the occurrence of knocking events. In a further preferred exemplary embodiment, upon transition into the knock prevention operating mode the so-called “fast advance” for the ignition angle is activated; in this, the adjustment angle is decreased either with a greater step size or at a greater frequency as compared to the advance otherwise performed. The adjustment angle is decreased until knocking is detected, which in turn results in a retarding of the ignition angle.

[0015] In a further preferred exemplary embodiment, the knock prevention with double injection operating mode is not activated until the respective present load value has exceeded a specific definable first load threshold value. In a further preferred exemplary embodiment, this first load threshold value is additionally dependent on engine speed, the first load threshold value being read out of a characteristic curve that is stored in the microprocessor of the internal combustion engine. In the characteristic curve of the first load threshold value, values for the first load threshold value are associated with individual engine speed ranges. A further exemplary embodiment of the invention concerns departure from the knock prevention with double injection operating mode. If the load falls below a second load threshold value, the microprocessor of the internal combustion engine performs a switchover into another operating mode. Such operating modes can be, for example, the homogeneous, homogeneous-lean, or stratified modes. These operating modes are sufficiently known for the direct gasoline-injection internal combustion engine, and therefore will not be described again. Various operating modes differ by having a fundamentally different configuration of important operating variables and underlying model concepts. Important operating variables in this context are the number of injection operations in one working cycle, the injection angle, the injection duration, the quantity of fuel injected and therefore the air ratio λ, and the ignition angle. One underlying model concept is, for example, the torque model. It is noteworthy for this exemplary embodiment that the second load threshold value lies below the first load threshold value, so that a hysteresis loop is traversed in terms of the load threshold value for the knock prevention operating mode. This prevents the internal combustion engine from attaining an unstable state between the knock prevention operating mode and another operating mode, and continually switching back and forth between the two operating modes. In a first exemplary embodiment, the second load threshold can be ascertained in such a way that a specific load threshold value difference is subtracted from the first load threshold value. Preferably this load threshold value difference can be stored in engine speed-dependent fashion in the microprocessor of the internal combustion engine. In a second exemplary embodiment, the second load threshold value can be read out directly in the microprocessor of the internal combustion engine from a characteristic curve that is engine speed-dependent, similarly to the first threshold value. The values of the second load threshold value are to be stored in the characteristic curve in such a way that they are smaller than the values of the first load threshold value.

[0016] In a further preferred exemplary embodiment, the knock prevention with exhaust gas recirculation operating mode is not activated until the respective present load value has fallen below a specific definable third load threshold value, since the exhaust gas recirculation action is used principally in low load ranges. In a further preferred exemplary embodiment, this third load threshold value is additionally dependent on engine speed, the third load threshold value being read out from a characteristic curve that is stored in the microprocessor of the internal combustion engine. In the characteristic curve of the third load threshold value, values for the third load threshold value are associated with individual engine speed ranges. A further exemplary embodiment concerns departure from the knock prevention with exhaust gas recirculation operating mode. If the load exceeds a fourth load threshold value, the microprocessor of the internal combustion engine then performs a switchover into another operating mode. Such operating modes can be, for example, in the case of direct gasoline-injection internal combustion engines, the homogeneous, homogeneous-lean, or stratified modes already mentioned; or in the case of internal combustion engines with manifold injection, operation without knock prevention. It is noteworthy for this exemplary embodiment that the fourth load threshold value lies above the third load threshold value, so that a hysteresis loop is traversed in terms of the load threshold value for the knock prevention operating mode. This prevents the internal combustion engine from attaining an unstable state between the knock prevention operating mode and another operating mode, and continually switching back and forth between the two operating modes. In a first exemplary embodiment, the fourth load threshold can be ascertained in such a way that a specific load threshold value difference is subtracted from the third load threshold value. Preferably this load threshold value difference can be stored in engine speed-dependent fashion in the microprocessor of the internal combustion engine. In a second exemplary embodiment, the fourth load threshold value can be read out directly in the microprocessor of the internal combustion engine from a characteristic curve that is engine speed-dependent, similarly to the third threshold value. The values of the fourth load threshold value are to be stored in the characteristic curve in such a way that they are smaller than the values of the third load threshold value.

[0017] In a further exemplary embodiment, the switchover into the knock prevention operating mode can be accomplished both in individual-cylinder fashion and simultaneously for all cylinders of the internal combustion engine. In the case of individual-cylinder switchover into the knock prevention operating mode, the adjustment angle of the respective cylinder is employed as the adjustment angle. If a switchover into the knock prevention operating mode is performed simultaneously for all cylinders, it is then advantageous to employ as the adjustment angle for the ignition angle the mean of the individual-cylinder adjustment angles for the ignition angle. Preferably, in this context, the arithmetic mean of the individual-cylinder adjustment angles is calculated. This calculation is in turn performed by the microprocessor of the internal combustion engine.

[0018] The provision of the knock prevention operating mode can result, because of an advancing of the ignition angle, in a torque gain and thus an increase in efficiency. The operating mode can easily be implemented for direct gasoline-injection internal combustion engines, since all that is necessary is to modify the adjustment angle and to apply an additional action reducing the knock susceptibility of the fuel/air mixture. 

What is claimed is:
 1. A method for operating an internal combustion engine, in which in at least one cylinder, combustion of a fuel in a fuel/air mixture introduced into the cylinder takes place, a baseline ignition angle in the at least one cylinder being retarded in steps upon the occurrence of knocking, using of a knock control system, to an adjustment angle from the basic ignition angle, wherein when the adjustment angle exceeds a definable threshold value: a) at least one additional action reducing the knock susceptibility of the fuel/air mixture is taken; and b) the adjustment angle is decreased so that the baseline ignition angle is advanced.
 2. The method as defined in claim 1, wherein the additional action reducing the knock susceptibility contains, for a direct gasoline-injection internal combustion engine, the fact that two fuel injection operations are performed in one working cycle.
 3. The method as defined in claim 2, wherein the quantity of fuel to be injected is divided among the two injection operations.
 4. The method as defined in one of claims 1 through 3, wherein the additional action reducing the knock susceptibility contains the fact that the exhaust gas recirculation rate is increased.
 5. The method as defined in one of claims 2 through 4, wherein when the definable threshold is exceeded, the additional feature of double injection is performed only above a definable first load threshold value, or the additional feature of exhaust gas recirculation is performed only below a third load threshold value.
 6. The method as defined in claim 5, wherein after the load has fallen below a definable second load threshold value for the additional feature of double injection, or after the load has exceeded a definable fourth load threshold value for the additional feature of exhaust gas recirculation, the internal combustion engine continues to be operated with a different operating mode, the second load threshold value being less than the first load threshold value and the fourth load threshold value being greater than the third load threshold value.
 7. The method as defined in one of claims 5 or 6, wherein the first load threshold value and/or the second load threshold value and/or the third load threshold value and/or the fourth load threshold value are dependent on engine speed.
 8. The method as defined in one of claims 1 through 7, wherein when the definable threshold value is exceeded, the adjustment angle is decreased quickly, the faster decrease being accomplished by way of a greater step size and/or a greater frequency of the decrease steps.
 9. The method as defined in one of claims 1 through 8, wherein the mean of the individual-cylinder retard angles of all cylinders is employed as the adjustment angle.
 10. The method as defined in claim 9, wherein the threshold is defined with the same value for all cylinders.
 11. The method as defined in one of claims 1 through 8, wherein the adjustment angle of the respective present cylinder is employed as the adjustment angle.
 12. The method as defined in claim 11, wherein the threshold value is defined in individual-cylinder fashion.
 13. The method as defined in one of claims 1 through 12, wherein the basic ignition angle is read out from a load-dependent and/or engine-speed-dependent characteristics diagram; and when the definable threshold value is exceeded, the basic ignition angle is read out from a load-dependent and/or engine-speed-dependent characteristics diagram differing from the other operating modes.
 14. An apparatus for operating an internal combustion engine having at least one cylinder, in which combustion of a fuel in a fuel/air mixture introduced into the at least one cylinder takes place, a microprocessor containing a knock control system being provided so that upon the occurrence of knocking, a baseline ignition angle in the at least one cylinder can be retarded in steps to an adjustment angle from the basic ignition angle, wherein if the adjustment angle exceeds a definable threshold value, means are provided in the microprocessor so that: a) an additional action reducing the knock susceptibility of the fuel/air mixture can be taken; and b) the adjustment angle can be decreased, so that the baseline ignition angle can be advanced.
 15. The apparatus as defined in claim 14, wherein for a direct gasoline-injection internal combustion engine, means are provided so that as the additional action reducing the knock susceptibility, two fuel injection operations can be performed in one working cycle.
 16. The apparatus as defined in claim 15, wherein means are provided in the microprocessor so that the quantity of fuel to be injected can be divided among the two injection operations.
 17. The apparatus as defined in one of claims 14 through 16, wherein means are provided so that as the additional action reducing the knock susceptibility, the exhaust gas recirculation rate can be increased.
 18. The apparatus as defined in one of claims 15 through 17, wherein means are provided in the microprocessor so that when the definable threshold is exceeded, the additional feature of double injection can be performed only above a definable first load threshold value, and/or if the definable threshold is exceeded, the additional feature of knock prevention with exhaust gas recirculation can be performed only below a third load threshold value.
 19. The apparatus as defined in claim 18, wherein means are provided in the microprocessor so that after the load has fallen below a definable second load threshold value for the additional feature of double injection, or after the load has exceeded a definable fourth load threshold value for the additional feature of exhaust gas recirculation, the internal combustion engine can continue to be operated with a different operating mode, the second load threshold value being less than the first load threshold value and the fourth load threshold value being greater than the third load threshold value.
 20. The apparatus as defined in one of claims 18 or 19, wherein the first load threshold value and/or the second load threshold value and/or the third load threshold value and/or the fourth load threshold value are dependent on engine speed.
 21. The apparatus as defined in one of claims 14 through 20, wherein means are provided so that when the definable threshold value is exceeded, the adjustment angle can be decreased quickly, the faster decrease being achievable by way of a greater step size and/or a greater frequency of the decrease steps.
 22. The apparatus as defined in one of claims 14 through 21, wherein means are provided in the microprocessor so that the mean of the individual-cylinder retard angles of all cylinders can be employed as the adjustment angle.
 23. The apparatus as defined in claim 22, wherein the threshold can be defined with the same value for all cylinders.
 24. The apparatus as defined in one of claims 14 through 21, wherein means are provided in the microprocessor such that the adjustment angle of the respective present cylinder can be employed as the adjustment angle.
 25. The apparatus as defined in claim 24, wherein the threshold value can be defined in individual-cylinder fashion.
 26. The apparatus as defined in one of claims 14 through 25, wherein means are provided in the microprocessor so that the basic ignition angle can be read out from a load-dependent and/or engine-speed-dependent characteristics diagram; and when the definable threshold value is exceeded, the basic ignition angle can be read out from a load-dependent and/or engine-speed-dependent characteristics diagram differing from the other operating modes. 